The present invention relates to weatherable coatings applied on exterior surfaces of flexible substrate articles, particularly elastomeric or rubbery articles or substrates containing such materials. In addition to providing protective film properties, the coatings reduce heat buildup by directing heat away from the article (emissive). The coatings can be applied to an elastomeric substrate either before or after the substrate has been vulcanized.
Engineered elastomeric products are designed to flex and bend, distort and recover, and/or dampen forces including absorbing torque or vibration repeatedly during their service life and are utilized in numerous industrial applications. For example, elastomeric materials are utilized in the manufacture of tires, hoses, seals, mountings such as engine mounts, dampers and insulating devices, and are designed to exhibit hysteretic losses, and withstand heat, to name a few design aspects. These and other articles shaped into myriad articles have many established uses such as industrial machines and parts for vehicles. Many elastomer products come into contact with heat from a variety of sources, such as from internal combustion engines. Recent increases in operating temperatures, and reduction of the size of vehicular engine compartments give rise to closer proximity between heat sources and such molded parts as rubber hoses, plastic housings, belts, various mounts, shrouds, seals, grommets, washers, spacers, covers, and housings, etc. Some of these articles are heat vulcanized, others are room temperature vulcanized and still others are cured in a different manner and exhibit characteristic flexing, elongation, rubbery elasticity, as thermoplastics or thermoset materials.
All polymeric materials degrade on account of exposure to heat, light, oxygen, ozone solvents, oils, and/or fuels. Elastomeric materials, and especially natural and/or synthetic vulcanized rubbers are particularly known to degrade when exposed to these agents, and there is a continuing search within industry to provide elastomer articles that are resistant to such degradative elements.
U.S. Pat. No. 6,022,626 discloses coatings suitable for covering engine mounts to protect the rubber substrate from oxygen, ozone and/or UV light, especially when reaching temperatures of 220xc2x0 F./104xc2x0 C., or more. The coatings taught provide a polymer barrier from chemical or UV intrusion. In exposure to hot environments, the polymers taught in U.S. Pat. No. 6,022,626 may provide an initial barrier against oxygen, ozone and UV radiation but lack durability to repeated flexure over long periods of time. Once adhesion fails or the coating is breached by cracks, degradative effects resume. Such coatings as taught in U.S. ""626 also do not provide emissive properties and do not deflect heat.
U.S. Pat. No. 5,314,741 to Roberts, et. Al. entitled xe2x80x9cRubber Article Having Protective Coatingxe2x80x9d relates to polymeric articles which are coated with hydrogenated synthetic rubbers or polymers obtained by hydrogenating an unsaturated polymer which is a polymer of 1,3-butadiene and optionally one or more monoethylenically unsaturated polymers.
Conventional polymeric stabilizers, UV absorbers and the like are used for the rubber articles coated thereon, yet improved aging properties are desired even in light of more harsh operating conditions.
Achieving sufficient permanent adhesion to the underlying rubber which experiences repeated flexure or extension over long-term service life is further needing improvement.
Alkyd, urethane, and enamel metallic paint finishes are well known for providing sparkled metallic effects, are widely used as on automotive bodies. The substrates are mainly metal or rigid plastic parts where flexure is limited or the paints are expected to crack if impacted severely. Speckled-effect metallic coatings are commonly provided on metal body panels, whereby 1% or less metallic pigments are interspersed with coloring pigments, and overcoated with clear finish. Likewise, aluminized spray paints have been provided for applying to furniture, metal articles and the like, however the film forming materials utilized, cure to form a coating of very limited elongation, and would be unsuitable as coatings on flexible substrates such as engineered rubber articles due to flex cracking and loss of adhesion not long after placing the coating in service. Metal flake effect paints provide visual aesthetics for appearance parts but do not provide heat emissive properties to any extent useful for extending the useful long term service of engineered rubber products under hot environments.
One method of rendering elastomeric materials resistant to corrosive materials is to apply a protective coating to the elastomeric material. Various corrosion-resistant coatings previously utilized for both flexible substrates (e.g., elastomeric substrates) and rigid substrates (e.g., steel, stainless steel, aluminum or plastic) include polyurethanes, polysulfides and fluorocarbon elastomers. When applied to rigid substrates, traditional corrosion-resistant coatings such as fluorocarbon elastomers have been found to provide excellent resistance to oil and fuel. However, when applied to flexible elastomeric substrates comprising natural rubber and/or diene-type elastomers and mixtures, the fluorocarbon elastomers suffer from poor fatigue resistance, poor low temperature characteristics, and poor adhesion to these substrates.
Low molecular weight polyolefin or polyisoolefin based elastomers containing a low level of chemically bound functionality such as an hydroxyl or an amine bearing group are known for incorporation into urethane foams. Such elastomers can be blended with and cured by an unblocked or blocked polyisocyanate. For example, U.S. Pat. No. 4,939,184 discloses the preparation of flexible polyurethane foams made by reacting a low molecular weight polyisobutylene having two or three terminal hydroxy groups with a polyisocyanate in the presence of a blowing agent.
U.S. Pat. No. 4,136,219 to Odam relates to two methods or processes for applying polyurethane paint to vulcanized rubber parts.
U.S. Pat. No. 4,670,496 discloses tire sidewall striping paint as a coloring indicia of any color, such as a dye, and preferably metallic particles are disposed in a solution that contains unvulcanized diene rubber(s) and rubber vulcanization accelerator. Crosslinkable silicone and/or modified EPDM may also be disposed in the solution. The accelerator is essential for scavenging sulfur from the vulcanized rubber substrate to provide auto-vulcanizing of the coating rubber. In order to provide adequate adhesion for long term service as a coating for rubber articles, a diene polymer containing more than 10% residual unsaturation after curing will necessarily undergo degradation and embrittlement and will fail long before the underlying substrate fails.
Diisocyanate containing free isocyanate groups has also been previously proposed for curing copolymers of isobutylene and modified styrene containing tertiary aminoalcohol groups in EPA 325 997. EPA 325 997 discloses diisocyanate curing of polymers having a molecular weight of 700 to 200,000, and exemplifies blends of up to about 30,000 weight average molecular weight (Mw) and about 8,600 number average MW (Mn), as measured by gel permeation chromatography.
A variety of bulk isocyanate-cured rubbers and mastics have been disclosed in the 50xe2x80x2s and 60xe2x80x2s. Isocyanate reactive functional groups present in the elastomer readily cure with NCO groups of the diisocyanate. As an example, U.S. Pat. No. 6,087,454 discloses a process to produce a cured bulk elastomer comprising combining an elastomeric polymer, having an Mw of 60,000 or more and containing hydroxyl and/or amine functional groups with a blocked polyisocyanate at a temperature below the temperature that will unblock the isocyanate. The mixture is cured by heating it to a temperature above the temperature that will unblock the polyisocyanate. This reaction can be effected at room temperature by the use of unblocked isocyanates. Low molecular weight polyisobutylene containing hydroxy functional groups are cured with a polyisocyanate in the presence of a blowing agent as is disclosed in U.S. Pat. No. 4,939,184.
U.S. Pat. No. 4,774,288 discloses a hydrogenated copolymer of a conjugated diene and an xcex1,xcex2-unsaturated nitrite containing an active phenol-formaldehyde resin vulcanization system. The disclosure is directed to the bulk vulcanizate, which is characterized as having good compression set properties and a good resistance to oils and good resistance to oxidative attack in air at elevated temperature aging under oxidizing conditions, however no mention is made suggesting coatings could be formed on flexible elastomeric substrates such as natural rubber and polybutadiene which might provide useful properties.
U.S. Pat. No. 5,314,955 discloses a coating composition consisting of (a) a hydrogenated acrylonitrile-butadiene copolymer, (b) a phenolic resin, (c) a curing component, and (d) a solvent. This coating solves many of the problems of adhesion to rubber substrates combined with fatigue resistance and fuel resistance. One of the drawbacks of this coating composition is that it requires a high temperature bake to cure the coating and to promote adhesion to adjacent metal surfaces. A high temperature baking conditions even for a coating requires heat soaking of the entire article to be coated. Some parts such as helicopter rotor bearings would be damaged by a high temperature bake, therefore coatings such as taught in ""955 are not practical to apply. The high temperature bake is also costly in production since it adds a time delay and additional handling of the parts. There still exists a need for improved protective coatings for flexible elastomeric substrates comprising typical natural rubber and/or diene-type elastomers that are resistant to fatigue over a broad temperature range, and that exhibit effective adhesion to the substrate, and that can be cured at room temperature if this is a limiting factor in coating an article.
U.S. Pat. No. 6,156,379 discloses a conventional base-coat-clear coat paint on metal surfaces, containing metal flakes in the base coat. The novel distinction is based on bright pigments derived from finely divided vapor-deposited metal. The metallic coating composition is applied over a base coating layer and a clear topcoating layer is applied over the metallic coating layer. A metallic coating composition is defined to consist essentially of the bright pigments and the solvent, meaning that coating composition either contains no ingredient other than the flake pigments and solvent, or a small amount of resin or additive such that the pigment weight concentration if 95% or higher. Binders such as acrylic, polyamide, vinyl chloride copolymers, urethane and polyesters are suggested. Such binders are not recognized as suitable for coating on flexible substrates as these can not exhibit 100% elongation, and will fail from flex-cracking and adhesion loss after placing in service.
U.S. Pat. No. 5,314,741 discloses a coating composition including a latex of highly saturated polymer such as hydrogenated nitrile rubber, highly saturated styrene/butadiene copolymer, hydrogenated polybutadiene, or hydrogenated styrene/vinyl pyridine/butadiene terpolymer. The coating is applied to a substrate and cured in place to yield a desired coated article reportedly resistant to ozone, oxygen, and UV light. Suitable curatives taught are zinc-sulfur cure packages. Elevated temperatures are necessary to affect curing of these coatings. Moreover, conventional vulcanizing systems high in sulfur content and low vulcanization accelerator content, or semi-efficient vulcanizing system having a moderate dosage of sulfur and vulcanizates accelerator known to the expert, and described e.g. in W. Hofmann, Kautschuk-Technologie, Genter Verlag, Stuttgart, 1980 p. 64 and 254-255 have several drawbacks. Conventional vulcanizing coatings result in vulcanizates with good resistance to dynamic stresses (flex life) are very sensitive to aging and reversion. Semi-efficient vulcanizing systems usually give vulcanizates which have a less of a resistance to dynamic stresses (flex life), but, in return, they are somewhat more stable to aging and reversion (cf. R. N. Datta and W. F. Helt, Rubber World, August 1997, p. 24, et seq.)
It has been observed by the present inventors that coatings based on highly saturated elastomers utilizing vulcanizing chemistry suffer from loss of adhesion to substrates such as blends of natural rubber and diene elastomers widely used in rubber articles in the aforementioned articles, especially on automotive tires, hoses and the like. A need still exists for an improved elastomeric protective coating for flexible elastomeric substrates which provide improved adhesion to the surface of elastomers, and improved flex-resistance as well as thermal emissive properties enabling the reduction of heat transferred to the underlying polymer substrate. The level of stress from heat under long-term service in engineered products is time and temperature dependant. Any reduction in absorbed heat and any increase in the release of heat within the elastomer can significantly extend the service/performance life of the product. It would be industrially important to decrease the rate of heat absorption, and increase the rate of heat dissipation of engineered elastomer products in order to extend the useful working life of these articles.
The present invention is directed to opaque, metal-filled emissive elastomeric coatings, devoid of rubber accelerator. The preferred embodiments are curable without heat. The coatings exhibit cured elongation of at least 100% and remain bonded to the substrate after long-term weathering. The coatings are in two parts which are mixed together at the time of application to the substrate. The first part comprises a flexible film-forming polymer exhibiting a Tg of less than 0xc2x0 C. and incorporated therein or thereon a functional group which is reactive to an active hydrogen containing curing agent, or the functional group is an active hydrogen-bearing group, and a liquid carrier. The film former polymer contains less than 10% ethylenic unsaturation before curing. The second or another part comprises a curing agent component containing either an active hydrogen bearing group and a crosslinking group, or the curing agent component contains a group reactive with active hydrogen and a crosslinking group, and a carrier liquid and (a) from 10 to 100 parts by weight per 100 parts by weight of film forming elastomer of thermally conductive metal particles having a particle size average of from 2 to 10 xcexcm or (b) from 20 to 150 parts by weight of thermal conductive particles having an average particle size of 20 to 60 microns.